This class implements the GActionGroup and GActionMap interfaces,
to let you add window-specific actions that will be exported by the
associated GtkApplication, together with its application-wide
actions. Window-specific actions are prefixed with the “win.”
prefix and application-wide actions are prefixed with the “app.”
prefix. Actions must be addressed with the prefixed name when
referring to them from a GMenuModel.

As with GtkApplication, the GDK lock will be acquired when
processing actions arriving from other processes and should therefore
be held when activating actions locally (if GDK threads are enabled).

The settings “gtk-shell-shows-app-menu” and
“gtk-shell-shows-menubar” tell GTK+ whether the
desktop environment is showing the application menu and menubar
models outside the application as part of the desktop shell.
For instance, on OS X, both menus will be displayed remotely;
on Windows neither will be. gnome-shell (starting with version 3.4)
will display the application menu, but not the menubar.

If the desktop environment does not display the menubar, then
GtkApplicationWindow will automatically show a GtkMenuBar for it.
This behaviour can be overridden with the “show-menubar”
property. If the desktop environment does not display the application
menu, then it will automatically be included in the menubar or in the
windows client-side decorations.

Handling fallback yourself

The XML format understood by GtkBuilder for GMenuModel consists
of a toplevel <menu> element, which contains one or more <item>
elements. Each <item> element contains <attribute> and <link>
elements with a mandatory name attribute. <link> elements have the
same content model as <menu>. Instead of <link name="submenu> or
<link name="section">, you can use <submenu> or <section>
elements.

Attribute values can be translated using gettext, like other GtkBuilder
content. <attribute> elements can be marked for translation with a
translatable="yes" attribute. It is also possible to specify message
context and translator comments, using the context and comments attributes.
To make use of this, the GtkBuilder must have been given the gettext
domain to use.

The following attributes are used when constructing menu items:

"label": a user-visible string to display

"action": the prefixed name of the action to trigger

"target": the parameter to use when activating the action

"icon" and "verb-icon": names of icons that may be displayed

"submenu-action": name of an action that may be used to determine
if a submenu can be opened

"hidden-when": a string used to determine when the item will be hidden.
Possible values include "action-disabled", "action-missing", "macos-menubar".

The following attributes are used when constructing sections:

"label": a user-visible string to use as section heading

"display-hint": a string used to determine special formatting for the section.
Possible values include "horizontal-buttons".